Flexible curtain rollup doors are used in many applications as exterior doors as well as in applications as interior doors between rooms wherein air pressure differentials may develop across the door. In this regard, it is known to provide flexible curtain or so-called rollup doors with moveable, transversely extending, and substantially rigid windbar members in both exterior and interior applications to minimize deflection of the door curtain under air pressure differential forces or so-called windloads. By way of example, the U.S. Pat. No. 5,632,317 issued May 27, 1997 to LeRoy G. Krupke et al. and assigned to the assignee of the present invention, includes description and claims directed to a moveable windbar for a rollup flexible curtain type door, which windbar is supported by opposed straps which are trained over drum members which rotate with the flexible curtain support drum. The windbar support straps are arranged such that the windbar retracts to a position directly under the door curtain in its rolled up condition when the door is open and the windbar extends or descends into a position approximately midway between the door curtain roller or drum and the door bottom edge in the closed position, thanks to a 2:1 reduction ratio in the overall movement of the windbar between open and closed positions.
However, in many applications of flexible curtain or similar rollup doors, a single windbar is not sufficient to prevent deflection of the door curtain to the extent which may cause the curtain to pull out of opposed side guide members. In this respect, at least one additional windbar disposed between the first mentioned windbar and the door bottom edge would be advantageous to minimize curtain pullout from the door side guides or unwanted release of the door bottom edge member in the event that the windload forces acting on the curtain became substantial. Accordingly, there has been a significant need for a windbar system for flexible curtain and similar so-called rollup type doors wherein at least two windbars are provided spaced apart between the door curtain drum and the door bottom bar when the curtain is in the closed position and which windbars are retractable out of the way to provide a suitable door opening when the flexible curtain is raised. It is to these ends that the present invention has been developed.